I am using a Celestron Evolution 8 with the Revolution Imager 2 camera. When previewing an image in SharpCap of a DSO (e.g., M57), the image is as expected - a dark (basically black) background with a faint image of the object. As soon as I start Live Stacking however, I get a bright, greenish background (like the attached shot) without having made any changes to anything. Adjusting the black level on the histogram helps some, but I can't get the black background back that SharpCap shows in the preview mode. I assume this is mostly noise from the camera gain control, but I don't understand the difference between what shows in the preview and what shows in the first frame of a LiveStack.

Live Stacking stretches (enhances) the darker areas of the image to improve visibility of faint objects. You can adjust this by moving the 'black level' and 'mid level' sliders in the 'Histogram' tab of the Live Stacking area - see http://docs.sharpcap.co.uk/3.2/#Histogram%20Tab

Thanks for the reply Robin. So Live Stacking immediately stretches the histogram? I must have missed that in the manual. I saw where it explains how a user can use the histogram tool to apply a stretch. I didn't notice that occurring in the tutorial videos I watched or in pictures I've seen posted where I assumed (I guess incorrectly) that the user had not made any adjustments to histogram settings.

As I had stated in my question, I realized that the histogram can be adjusted, but I was having trouble getting the background back to as dark as it had looked in the preview. I guess I just need to work at it a little more.

Thanks for the explanation; I now understand better why it looks the way it does, and at least conceptually what I need to do to try to "improve" the image.

That is a pretty decent image - the faintest parts of M27 can actually be seen. I have an LN300 style CCTV camera and I never got that much with M27 (that was in the early days). You might have been caught out by the power of live stack in that your exposure/gain settings might be too high. The stack will compensate for lower settings by the build-up of frames. Maybe post your capture settings file and people might be able to offer settings to try for the Imager 2. Have a look at https://docs.sharpcap.co.uk/3.2/#The%20Histogram for some background on the histogram.

In the forum, search for mAnKiNd live stack, Minos is the master at this. The posts to look at are in the Gallery sub-forum.

The Live Stack videos you are finding on the web will be older and the features discussed only appeared in v3.2 of Sharpcap - hence the lag.

Thanks Dave. And thanks for the link to the histogram page. I had read all that beforehand, but promptly forgot most of it while trying to capture the image! I will have to slow down and develop a procedure to take advantage of all the options. I think it will be mostly a matter of just doing it, experimenting and gaining enough experience to have it become second nature.

One issue regarding the capture settings is that the camera cannot be controlled by SharpCap so the camera settings text file produced by SharpCap does not show the actual camera settings. I am pretty sure that I used a 5 sec exposure and a max gain of 30 dB. I also used the brightness image control in the SharpCap camera control menu to get a good image on the screen, but had forgotten that those settings are not reflected in the saved (save as viewed) image. I am not quite sure how that works since I think it affects the histogram and my selection of histogram settings are based on that view.

I am saving the stack (not an individual frame) as a png file. So it should correspond with the histogram. Where i am confused is that the camera control brightness affects the black level and gamma I choose in the histogram, yet the camera control brightness is not included in the saved png. So I may I be misled in choosing black level and gamma if I am expecting the png to look like the screen? If so, what camera control brightness level is being used for the saved png file.

In perusing the version 3.2 documentation, it looks like there is a new save option that will include the camera controls effects in the saved png so it looks exactly like what is on the screen. This may resolve my confusion.

Here are a couple of other images of M27 that I saved the same night using different black level and gamma settings. I am having trouble choosing which one is "best." I tend to like the image with the darker background, but then I'm not sure it shows as much of the fainter parts of the nebula.

The attached pdf is a rough description of the steps taken. Try the steps on another of your images and let me know if the material was any use. If anything is not clear get back to me and I will amend the doc.

Wow. I don't know how to express my gratitude. Thank you so much. I had just started to play with this image in GIMP as well, but was only using the brightness/contrast tool and curves and was not satisfied with my result. I had just resigned myself to studying the GIMP user manual to learn how to use it properly (I had just downloaded GIMP a couple of days ago to try to improved these images). While I still intend to do that, you're instructions were exactly what I needed!

I just have a couple of questions. For the blue channel adjustment on page 7 of your instructions, it looks like you also changed the output level to 60. Did you do this before adjusting the input level to 86 or after? How did you choose this adjustment level? When changing the output level, the changes to the histogram are not "smooth" changes like when changing the input level, so it seems like choosing a combination of input/output levels might be an iterative process?

I am unsure whether I fully understand the scaling process. You suggesting scaling the image by 50% because it results in 'tighter' stars. I agree it looks better, but it is also a much smaller image. Later, if I open the final png image and zoom in on it to make it bigger, doesn't this just undo the image scaling that was done in GIMP? Or is there something fundamentally different between "scaling" and "re-sizing" an image? Are there "preferred" sizes based on the resolution of the camera used?

You noted the possible reasons for the stacking artefacts. I am using a Celestron Evolution 8, which has an alt-az mount. I did not do any polar alignment, just the 3-star alignment using SkySafari. I know that with longer exposures there will be frame rotation issues even with the stacking alignment process, but I thought that a 2-minute exposure like this one would would have minimal such issues. At least if I can properly center the image, I am prepared to crop any such artefacts!

I made the mistake of finding your beautiful image of M27 in the forums. The color difference is striking as is the sharpness/clarity. Would you recommend saving the SharpCap live capture as a RAW FITS file and use other software for further processing (e.g., Deep Sky Stacker, Nebulosity), or using the tools in SharpCap and saving the as viewed png file as I did?

The other difference I noticed is that my picture appears rotated compared to yours. I have seen images on the Internet that appear to have various amounts of rotation compared to my image. I have to admit I am not sure which end is "up" with my camera and I did not pay a lot of attention to how it was inserted in the diagonal in the dark at the time. I am also not sure whether it should be inserted "upside down" to take into account the inverted image from the diagonal or whether the image should be oriented upside down like the view you would get from an eyepiece.

Thanks again for your brilliant tutorial. I don't know how I could ever repay you for that!